1. Field of the Invention
This invention relates generally to a boresight thermal reference source used to provide a uniform, high intensity Long Wave Infra-Red (LWIR) beam within the 7.5-12 .mu. waveband in order to assure boresight alignment of laser and Forward-Looking Infra-Red (FLIR) sensor lines-of-sight. More particularly, the invention relates to a boresight thermal reference source consisting of a ceramic rod heated by a nichrome wire partially wrapped around the ceramic rod, creating a pseudo-blackbody cavity. This invention is equally applicable at other infrared wavebands, specifically the 3-5 .mu.m waveband.
2. Description of the Related Art
The boresight thermal reference source of the present invention is used in a laser designation and thermal imaging system currently known as the AESOP program in order to permit operator initiated auto-alignment of a laser to a FLIR sensor, necessary to accurately track, lock on and fire missiles at targets. The high beam power, provided by the boresight thermal reference source, is required because the reflected boresight thermal reference source aperture at the FLIR entrance aperture subtends only 1/346 the area of the FLIR entrance pupil.
Other presently available heat sources (heat plates, halogen light bulbs, etc.) cannot become hot enough to provide the desired IR signal. CO.sub.2 lasers are much too large and are very expensive. IR laser diodes are impractical because they require cooling down to 77.degree. K. Globars are too large and require a high amount of power. While halogen light bulbs were used previously for similar applications, the temperature of a halogen bulb envelope (about 120.degree. C.) is significantly less than that provided by the present boresight thermal reference source and less than that required in applications like the AESOP system.
The ceramic material used in the preferred embodiment of the present invention is Macor, which is easily machined. Utilization of a heated nichrome wire by itself, without the ceramic rod, lacks sufficient uniformity and emissivity for proper use. The only way to provide more heat (i.e., IR beam power) than the present design is to use higher temperature ceramics, which are more difficult to machine than Macor, and a tungsten heater wire, requiring a vacuum enclosure with a Long Wave Infra-Red (LWIR) window. This latter design, however, would be significantly more expensive than the present invention.